Compared to other compact laser sources, fiber lasers present superior performance in terms of spectral purity and noise. Furthermore, their output is readily compatible with fiber optics systems and components by fusion splicing and standard connectors. Nowadays fiber lasers with high spectral purity are typically realized using photo-induced fiber Bragg gratings.
In recent years, optical fiber lasers have been developed to cover a wide range of spectral bands. The gain medium of these lasers is typically composed of silica, fluoride or chalcogenide host matrix doped with rare earth ions. Of particular interest is erbium-doped silica which produces gain in the 1530 to 1610 nm wavelength band. This spectral region, also known as the third communication window, corresponds to the minimum loss of silica optical fiber.
Fiber lasers can operate either in continuous wave (CW) or pulsed (Q-switched or mode-locked) emission regimes. In the latter regime, the advantages of fiber lasers are its high peak powers, energies and repetition rates. In the former regime, fiber lasers are attractive because of their narrow linewidth and spectral tunability. In both cases, other advantages include the compactness of the laser source and the compatibility of the laser output to optical fiber transmission link and components by direct fusion splicing. A review of fiber laser technology can be found in M. J. F. Digonnet, editor, Rare-Earth-Doped Fiber Lasers and Amplifiers, Marcel Dekker, 2001. The content of the above document is incorporated herein by reference.
Narrow line-width single-mode fiber lasers operating in CW regime can be made using several configurations. In free running mode, the emission wavelength corresponds to the wavelength having the highest gain. To tune the emission wavelength, a narrow-bandwidth filter can be incorporated in the cavity. Initially, fiber lasers had long cavities that resulted in a highly multimode spectrum at the emitting wavelength. To obtain single-mode emission, complex configurations involving either coupled cavities or a cascade of narrow filters had to be used. In all cases, the lasers required extensive stabilization systems. Recent progress in the development of fiber Bragg gratings has allowed the realization of short fiber lasers with single-mode output. These lasers are easier to stabilize than the previous configurations. Furthermore, the emission wavelength can be varied by temperature or strain tuning of the fiber gratings.
In many applications, it is required to have a laser source emitting on several wavelengths or frequencies. In telecom applications, these frequencies are usually spaced by fixed intervals like 50 GHz, 100 GHz or 200 GHz. At each of these frequencies, the emission spectrum has to be very pure. Fiber lasers are usually not considered to be good candidates for multi-frequency laser sources because the gain competition between the lasing frequencies results in an unstable output that allows emission over only a few closely spaced wavelengths. This effect is intrinsic to rare-earth doped silica materials which typically behaves like homogenously broadened gain medium at room temperature.
Fabry-Perot laser cavities are realized by placing a gain medium between two mirrors. These cavities are characterized by resonance frequencies, known as longitudinal modes, spaced by:Δf=c/2nL  (1)where Δf is the frequency separation between two modes, n the refractive index of the medium, L the length of the cavity, i.e the distance between the reflectors, and c the speed of light. Long cavities will therefore have closely spaced modes. To reduce the number of lasing modes, it is necessary to introduce some differentiation in the net gain, i.e. spectral gain of the medium minus the spectral cavity loss, experienced by the longitudinal modes. Similarly to semiconductor technology this mode selection is accomplished with a narrow band reflector made by a distributed index modulation. For additional information, the reader is invited to refer to G. Mothier, P. Vankwikelberge, Handbook of distributed feedback laser diodes, Artech House, 1997 and H. Kogelnik and C. V. Shank, “Coupled-Wave Theory of Distributed Feedback Lasers”, J. of Appl. Physics 43, pp. 2327-2335 (1972). The content of the above documents is incorporated herein by reference. These are known as Bragg gratings. A modulation of the refractive index with a period A creates a narrow-band reflector centered on the Bragg wavelength defined by:λB=2neffΛ  (2)where neff is the effective index of the reflected waveguide mode. The maximum reflection and the bandwidth of the distributed mirror are related to the amplitude of the index modulation as well as to the length of the grating. For example, a review of fiber Bragg grating technology that can be photoinduced in optical fibers or glass waveguides by exposure to UV light can be found in R. Kashyap, Fiber Bragg Gratings, Academic Press, 1999. The content of the above document is incorporated herein by reference.
Two types of narrow linewidth laser configurations are typically used. The first one is the DBR laser (Distributed Bragg Reflector) and the second one is the DFB laser (Distributed Feedback). In the first type, represented in FIG. 1, the short gain section is sandwiched between two narrow-bandwidth reflectors. The effective length of the cavity corresponds to the spacing between the gratings plus a penetration depth that takes into account the dispersion and time response of the distributed reflectors. The longitudinal mode spacing of the cavity is such that only one mode will resonate with sufficient gain and therefore singlemode output is obtained. In the second type, represented in FIG. 2, a ρ phase shift is introduced in the grating structure. The structure then presents only one resonating mode with a frequency corresponding to the Bragg wavelength.
DFB or DBR fiber lasers also make use of several external components. With reference to FIGS. 3a and 3b, the doped optical fiber 300 with the laser structure is typically fusion spliced to a wavelength selective coupler (WDM coupler) 302 on one end, to allow the injection of the pump laser 304 light, and to an isolator 306 on the other end, to avoid instabilities caused by reflections. For erbium-doped glasses, the pump 304 is more often a 980 nm laser diode but sometimes also a 1480 nm laser diode will be used. The injection of the pump 304 can be performed either in a co-propagation configuration or a counter-propagation configuration. FIG. 3a shows the different components of a co-propagation pumping configuration and FIG. 3b shows the different components of a counter-propagation pumping configuration.
Most work on fiber laser has been done using silica optical fibers doped with Er3+ to obtain an output wavelength in the 1530 to 1560 nm spectral range. Because of the short length of the gain section, the fiber is often co-doped with Yb3+ ion to increase the pump absorption. In all cases, high-doping concentration of Er3+ and Yb3+ is typically used to provide sufficient absorption and gain. At the same time, the photosensitive response of the optical fiber must be sufficient to allow the writing of the refractive index grating acting as the reflectors. The photosensitivity of optical fiber is most often related to the presence of the germania dopant used to increase the refractive index of the fiber core. Unfortunately, high levels of germania dopants leads to clustering of the rare earth ions and reduces the emission properties of fiber lasers. In 1997, researchers at Southampton University introduced a new design of optical fiber in which the rare earth ions are introduced in the core of the optical fiber while germania and boron are present in an annular region of the cladding. This is described in L. Dong, L., W. H. Loh, J. E. Caplen, J. D. Minelly, K. Hsu and L. Reekie “Efficient single-frequency fiber lasers with novel photosensitive Er/Yb optical fibers”, Opt. Lett. 22, pp. 694-696 (1997). The contents of this document are incorporated herein by reference. This fiber design, separating the active and the photosensitive regions, has allowed higher output power to be obtained for both DBR and DFB configurations. Another way to increase the photosensitivity of optical fiber is to use hydrogen loading techniques such as those described in P. Lemaire, R. M. Atkins, V. Mizrahi, W. Reed, “High pressure H2 loading as a technique for achieving ultrahigh UV photosensitivity and thermal sensitivity in GeO2 doped optical fibres.” Electron. Lett. 29, pp. 1191-1193 (1993). The contents of this document are incorporated herein by reference. In such techniques the fiber is placed under high pressure of hydrogen to make the hydrogen molecules diffuse in the glass. Upon 10 exposure to UV radiation, the photosensitive response is greatly enhanced but optical losses of typically 0.1 dB/cm will also be induced around 1550 nm as shown in D. Johlen, F. Knappe, H. Renner, and E. Brinkmeyer, “UV-induced Absorption, Scattering and Transition Losses in UV Side-Written Fibers”, OFC paper ThD1-1, p.50-52 (1991). The loss is due to an overtone of a vibration mode of the OH group that is being formed in the fiber. This loss is very detrimental to the performance of short single-mode fiber lasers. A possible solution is to replace the hydrogen by deuterium for which the vibration mode is located at higher wavelengths as described in J. Stone, “Interactions of Hydrogen and Deuterium with Silica Optical Fibers: A Review”, J. of Ligthwave Technol. LT-5, pp.712-732 (1987). The contents of the above document are incorporated herein by reference.
The first single-frequency single-mode fiber lasers were DBR lasers with 1-2 cm cavity length described in:                G. A. Ball and W. H. Glenn, “Design of a single-mode linear-cavity erbium fiber laser utilizing Bragg reflectors”, J. Lightwave Technol. 10, pp. 1338-1343 (1992).        G. A. Ball, W. H. Glenn, W. W. Morey, and P. K. Cheo, “Modeling of short, single-frequency, fiber lasers in high-gain fiber”, IEEE Photon. Technol. Lett. 5, pp. 649-651 (1993).        J. L. Zyskind, V. Mizrahi, D. J. DiGiovanni and J. W. Sulhoff, “Short single frequency erbium-doped fibre laser”, Electron. Lett. 28, pp. 1385-1387 (1992).        
The contents of the above documents are incorporated herein by reference. The gratings were photo-induced in an Er3+ doped silica optical fiber and presented output power of typically 50-100 μW. In later work, an amplification section was added after the single-mode laser to reach higher output power, typically 3-10 mW as described in G. A. Ball and W. W. Morey, “Compression-tuned single-frequency Bragg grating fiber laser”, Opt. Lett. 19, pp. 1979-1981 (1994) and J. -M. Delavaux, Y. -K. Park, V. Mizrahi, and D. J. DiGiovanni, “Long-term bit error rate transmission using an erbium fiber grating laser transmitter at 5 and 2.5 Gb/s”, Opt. Fiber Technol. 1, pp. 72-75 (1994). The contents of the above documents are incorporated herein by reference.
Ball et al. also demonstrated wavelength tuning over 32 nm by compression of the fiber laser along its axis. Such a DBR fiber laser was used as the optical source for a transmission experiment at 5 and 2.5 Gbit/s. The efficiency of DBR lasers was later improved to 25% using an Er3+/Yb3+ co-doped fiber with photosensitive cladding and output power in excess of 15 mW was obtained without amplification.
DFB fiber lasers were first realized by writing uniform fiber Bragg gratings over an Er3+ doped silica optical fiber. The phase-shift was subsequently induced either temporarily by heating the fiber or permanently by performing a second UV exposure of a small section of the grating. Such DFB fiber lasers are described in J. T. Kringlebotn, J. -L. Archambault, L. Reekie, and D. N. Payne, “Er3+:Yb3+-codoped fiber distributed-feedback laser”, Opt. Lett. 19, pp. 2101-2103, (1994) and M. Sejka, P. Varming, J. Hübner and M. Kirstensen, “Distributed feedback Er3+-doped fibre laser”, Electron. Lett. 31, pp. 1445-1446 (1995). The contents of the above documents are incorporated herein by reference.
In later work, the phase-shifted grating was written in a single step as described in W. H. Loh, and R. I. Laming, “1.55 μm phase-shifted distributed feedback fibre laser”, Electron. Lett. 31, pp. 1440-1442 (1995). Typical output powers of the devices are 1-3 mW in usual photosensitive Er3+/Yb3+ co-doped fibers to 10-20 mW in Er3+/Yb3+ co-doped fibers with photosensitive cladding.
Although most devices are found to operate on a single longitudinal mode, two polarization modes separated by a few GHz are often observed. The splitting of the polarization modes is caused by intrinsic or photoinduced birefringence in the optical fiber cavity. Truly singlemode behavior requires the suppression of one of the polarization modes. Single polarization emission is usually obtained by increasing the birefringence of the optical fiber either through UV exposure or by applying external strain to the fiber like transverse strain or twist. Both these methods have been used to obtain single frequency fiber laser. For additional information, the reader is invited to refer to the following documents:                E. Ronnekleiv, M. N. Zervas, and J. T. Kringlebotn, “Modeling of Polarization-Mode Competition in Fiber DFB Lasers”, IEEE J. Quantum Electron. 34, pp. 1559-1569 (1998).        Z. E. Harutjunian, W. H. Loh, R. I. Laming, and D. N. Payne, “Single polarisation twisted distributed feedback fibre laser”, Electron. Lett. 32, pp. 346-348 (1996).        H. Y. Kim, S. K. Kim, H. J. Jeong, H. K. Kim, B. Y. Kim, “Polarizarion properties of a twisted fiber laser”, Opt. Lett. 20, pp.386-389 (1995).        H. Storoy, B. Sahlgren, and R. Stubbe, “Single polarisation fibre DFB laser”, Electron. Lett. 33, pp. 56-58 (1997).        M. Ibsen, E. Ronnekleiv, G. J. Cowle, M. O. Berendt, O. Hadeler, M. N. Zervas, and R. I. Laming, “Robust high power (>20 mW) all-fibre DFB lasers with unidirectional and truly single polarisation outputs”, Technical Digest of the Conference on Lasers and Electro-Optics (CLEO), paper CW4, pp.245-246 (1999).        
The contents of the above documents are incorporated herein by reference.
Multi-frequency operation of fiber lasers on well-separated wavelengths is usually prevented by cross gain saturation. Generally speaking, erbium-doped silica behaves at room temperature as a homogenously broadened gain medium. Therefore, simultaneous emission can usually be observed over a few, typically two or three, closely spaced wavelengths provided that the spectral gain is very flat. Emission over a larger number of wavelengths can be achieved by cooling the fiber to a cryogenic temperature, a rather unpractical approach. Another solution is to spatially separate the sections of the gain medium with which the different wavelengths interact. In the following paragraphs, we present in details some of these solutions.
The first approach is based on a miniature FP (Fabry-Perot) laser with a cavity composed of a short segment of doped fiber 400, 1 or 2 mm, placed between external reflectors 402 404 as shown in FIG. 4a. The longitudinal mode spacing of this laser, Δf of 100 GHz or 50 GHz, corresponds to the desired frequency comb. By immersing the laser in liquid nitrogen multi-frequency emission is obtained over 17 wavelengths as depicted in FIG. 4b. However, as the temperature is increased, gaps appear in the optical spectrum and the power becomes unstable as depicted in FIGS. 4c and 4d. In all cases the output power is very low, typically of the order of 100 μW for all the wavelengths combined.
In another approach, depicted in FIG. 5a, individual DFB or DBR fiber 500-508 are placed in series along an optical fiber and pumped with a common pump source 510. This configuration is described in S. V. Chernikov, J. R. Taylor and R. Kashyap, “Coupled-cavity erbium fiber lasers incorporating fiber grating reflectors”, Opt. Lett. 18, pp. 2023-2025 (1993) and J. Hübner, P. Varming and M. Kristensen, “Five wavelength DFB fibre laser source for WDM systems”, Electron. Lett. 33, pp. 139-140 (1997). The contents of the above documents are incorporated herein by reference. Hübner et al. have thus realized a five-wavelengths laser source having an output spectrum of the type shown in FIG. 5b. Each fiber laser being 5 cm long, the resulting structure is therefore quite long and difficult to stabilize.
Another approach, depicted in FIG. 6a, is to multiplex a plurality of individual DFB lasers 600 using wavelength selective couplers or combiners. The implementation presented in FIG. 6 also includes pump combiners 606 to provide protection against the failure of a laser pump. Although the available output power obtained from each laser is high, approximately 3 mW, the number of pump laser diodes increases the cost and complexity. FIG. 6b shows the output spectrum (optical power in dBm vs wavelength in nm) of the configuration of FIG. 6a. 
A last approach is to realize multiple DFB lasers on the same fiber segment for example by writing two gratings with slightly different period. This grating structure, also known as a Moire grating, is represented in FIG. 7a. FIG. 7b shows the output spectrum for several dual-frequency laser samples with different frequency spacing. A specific implementation of this technique was described in M. Ibsen, E. Ronnekleiv, G. J. Cowle, M. O. Berendt, O. Hadeler, M. N. Zervas, and R. I. Laming, “Robust high power (>20 mW) all-fibre DFB lasers with unidirectional and truly single polarisation outputs”, Technical Digest of the Conference on Lasers and Electro-Optics (CLEO), paper CW4, pp.245-246 (1999). The content of the above document is incorporated herein by reference. In this document, the grating structure was written in a single step and one of the phase shifts was omitted to create the laser cavity. Emission over two wavelengths was obtained. This dual wavelength emission was attributed to spatial hole burning in the laser cavity. This approach is however limited in terms of the number of wavelengths that could be achieved since all the laser cavities are located at the same position on the optical fiber.
In the context of the above, there is a need in the industry to provide a multi-wavelength laser source that alleviates at least in part problems associated with the existing methods and devices.